AIRS Level 1B Visible/Near-Infrared Channels ATBD - NASA's Earth ...

AIRS Level 1b ATBD Part 2: Vis/Near IR Channels
The spectral response curve of the visible channels is defined by the product of the spectral
response of these elements:
4. The reflectivity of three mirrors (the scan mirror and two fold mirrors).
5. The transmission characteristics of the field imaging lenses.
6. Sharp cut-on and cut-off interference filters (except for the long-wave side of channel 4,
which uses a pass-filter).
7. The spectral response curve of the Silicon detectors.
Measurements have been made of the spectral response of all the above items (Hofstadter
1999a and Fig. 1.1), except the lens transmission coefficients. The lenses are not expected
to have an appreciable spectral effect, however, and can be ignored in this regard. The scan
mirror and the folding mirrors of the optics are Silver coated with a thin layer of SiO x
, using
Denton proprietary process FSS99. This coating should have a minimal (< 2%) polarization
dependence in the 0.4 to 1.0 µm region, which was too small to detect by the test set-up used
during ground characterization of the scan mirror.
The long wavelength spectral extent of Channel 4 is controlled by the cutoff wavelength of
the Si diode, which is radiation dose dependent. Figure 1.2 shows the relative response of
the Si diode before radiation exposure (pre-rad) and after five years in orbit (post-rad). The
spectral response of Channel 4 (and, to a lesser extent, Channels 2 and 3) therefore changes
slowly and proportionally to the total radiation dose received. There is no in-orbit capability
to directly verify the spectral passband. An empirical spot check validation of the spectral
response curves using various ground test sites and algorithms in support of this validation
are discussed in Section 3.
1.0
0.8
Pre-rad
Post-rad
0.6
0.4
0.2
0.0
0.4 0.6 0.8 1.0
wavelength (µm)
Figure 1.2: The effect of radiation on the spectral response curve of the Si diode. The upper curve (marked
“Pre-rad”) shows the expected response at launch. The lower curve (“Post-rad”) is the expected response after
five years in orbit. These data were provided by LMIRIS.
10 November 2000 Version 2.2 7